Cryostat device

ABSTRACT

IN A CRYOSTAT HAVING A HELIUM REPLENISHING BATH SEPARATED FROM A HELIUM WORKING BATH BY A FIRST FILTER ELEMENT MADE OF A MATERIAL WHICH IS AT LEAST PARTIALLY IMPERMEABLE TO GASEOUS HELIUM AND HELIUM I, AS WELL AS TO ANY HELIUM II HAVING A TEMPERATURE HIGHER THAN THAT OF THE WORKING BATH, A SECOND FILTER ELEMENT OF SUCH MATERIAL SUPPORTED IN THE REPLENISHING BATH CONTAINER SPACED FROM AND ABOVE THE FIRST FILTER ELEMENT. THE SECOND FILTER ELEMENT FORMS THE NORMAL FLOOR OF THE REPLENISHING BATH. A SPACE BETWEEN THE FIRST AND SECOND FILTER ELEMENTS IS IN COMMUNICATION WITH THE REPLENISHING BATH CONTAINER PORTION ABOVE THE REPLENISHING BATH, WHEREBY THERE IS SUBSTANTIALLY NO PRESSURE DIFFERENCE ACROSS THE SECOND FILTER ELEMENT SO THAT THE POSSIBILITY OF PRESSURE-CAUSED BREAKTHROUGH OF THE REPLENISHING HELIUM INTO THE WORKING BATH IS ELIMINATED.

Nov. 16, 1971 CRYOSTAT DEVICE Filed March 17, 1970 A. ELSNER EPA!-INVENTORS. Albrecht Eisner Gus-10v Klipping ATTORNEYS.

United States Patent 3,620,033 CRYOSTAT DEVICE Albrecht Elsner,Hechtsheim, and Gustav Klipping, Berlin, Germany, assignors toMax-Planck-Gesellschaft Zur Forderung der Wissenschaften, e.V.,Gottingen, German Filed Mar. 17, 1970, Ser. No. 20,163 Claims priority,application Germany, Mar. 19, 1969,

P 19 13 789.4 Int. Cl. F17c 7/02 US. Cl. 6255 4 Claims ABSTRACT OF THEDISCLOSURE In a cryostat having a helium replenishing bath separatedfrom a helium working bath by a first filter element made of a materialwhich is at least partially impermeable to gaseous helium and helium I,as well as to any helium II having a temperature higher than that of theworking bath, a second filter element of such material supported in thereplenishing bath container spaced from and above the first filterelement. The second filter element forms the normal floor of thereplenishing bath. A space between the first and second filter elementsis in communication with the replenishing bath container portion abovethe replenishing bath, whereby there is substantially no pressuredifference across the second filter element so that the possibility ofpressure-caused breakthrough of the replenishing helium into the workingbath is eliminated.

CROSS-REFERENCE TO RELATED PATENT The present invention relates to animprovement of the invention set forth in US. Pat. No. 3,442,091, issuedMay 6, 196 9, to Gustav Klipping, Albrecht Elsner, and Gerd Hildebrandtfor Delivery of Coolant to Cryostats.

BACKGROUND OF THE INVENTION US. Pat. No. 3,442,091 concerns a device forcontinually cooling in a liquid bath, especially at temperatures below2.17 K., using a He II bath. Temperature regulation is obtained bycontrolling the pressure in the vapor chamber above the helium bath.Replenishment is controlled as a function of the level of the heliumbath. The outlet of a vacuum-jacket syphon opens into a replenishmentchamber which is under reduced pressure. Helium is fed intermittentlyinto the replenishment chamber through the vacuum-jacket syphon. Thereplenishment chamber is separated from the chamber containing thehelium bath by a filter element which is at least partially impermeableto gaseous helium, helium I, as well as to helium II of highertemperature than the tempera ture of the helium II bath.

An apparatus according to US. Pat. No. 3,442,091 makes it possible toreplenish a helium working bath held at temperatures below the k-pointby reduced pressure, while maintaining the working bath temperatureconstant. Such working baths are used for cooling objects. Suchapparatus makes it possible to continually cool objects at temperaturesbelow 2.17 K.

To replenish the working bath, liquid helium, boiling at atmosphericpressure (4.2 K.), is brought from a storage vessel through a syphoncooled by exhaust gas. The syphon carries on its end an expansion valvesurrounded by a sintered body or the equivalent. This helium passesthrough the expansion valve into a replenishment chamber held underreduced pressure and expands and cools. The filter is practicallyimpermeable for this replenishing bath so long as the temperature of thereplenishing bath 3,620,033 Patented Nov. 16, 1971 SUMMARY OF THEINVENTION It has been discovered that if the pressure diiference acrossfilter of UJS. Pat. No. 3,442,091 exceeds a certain maximum valuedependent on the pressure below the filter, then liquid helium begins tollow from the replenishing bath, through the filter, into the workingbath, even though the temperature of the replenishment bath is abovethat of the working bath. The pressure below the filter of course,corresponds to the particular working temperature of the working bath.At a temperature of the working bath of 1.5 K., which corresponds to apressure below the filter of 5 millibars, such liquid breakthroughoccurs at a pressure difierence of 650 millibars. As another example, ata working bath temperature of 2.1 K., which corresponds to a pressurebelow the filter of 47 millibars, liquid breakthrough occurs at 50millibars.

As is evident from the two examples of the last paragraph, liquidbreakthrough begins at increasingly higher pressure differences as thetemperature of the working bath decreases. While in general thisbreakthrough does not lead to a change of the temperature in the workingbath, it can have a disturbing efi ect on the steady state operation ofthe apparatus, since the pump attached to the space above the workingbath is subjected to an undesirably high loading.

An object of the present invention, therefore, is to prevent liquidbreakthrough at higher pressure differences across the filter.

This as well as other objects which will become apparent in thediscussion that follows are achieved by the feature that thereplenishment chamber of US. Pat. No. 3,442,091 is provided with atleast one filter additional to that shown in that patent. Thisadditional filter is spaced above the first filter element and the spacebetween the first filter element and this second filter element is incommunication with the portion of the replenishment chamber above thesecond filter element.

With the double filter of the present invention, the phenomenon ofliquid breakthrough is prevented with certainty, since the pressuredifference across the upper filter element (whose job it is to supportthe replenishment bath) is always zero. The gravitational pressure ofthe liquid helium, which is yet acting on the upper filter, isnegligibly small: gh':3 mb. for a height of the liquid of 200 mm. On theother hand, liquid flow through the double filter upon reaching or goingbelow the working temperature of the working bath still proceeds as inthe US. Pat. No. 3,442,091, since the blocking elfect of both filters islost at the same time. The liquid flows through the upper filter intothe space between the filters and from there through the lower filterinto the working bath below.

BRIEF DESCRIPTION OF THE DRAWING The sole figure of the drawing is across-sectional view of the cryostat of the invention, the plane of thecross section containing the cylindrical axis of the cryostat.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Since the present invention isan improvement of that disclosed in US. Pat. No. 3,442,091, only so muchof the old structure as will sufiice to show the connection of thepresent invention is illustrated in the single figure of the drawing.The details of the remainder of the old structure are available in theabove-mentioned patent and incorporated herein by reference.

Referring now specifically to the figure, inner container 1 of thecryostat of the present invention holds a helium II working bath 2. Theradiation protection, which surrounds the inner container, and which iscooled with liquid nitrogen, has been omitted from the drawing, sinceits illustration is not required by one skilled in the art for a properunderstanding of the invention.

The inner container 1 is provided with a cover 3 which carries acylindrical partition 4 which forms the replenishment chamber of thepresent apparatus. This partition holds the helium replenishing bath 5and is closed at its lower end by a fine-pored filter element 6. Thefilter element 6 can be bypassed by a valve 7 located in the floor ofthe partition 4- adjacent the filter element 6. Above the filter element6 and within partition 4 is provided a second filter element 8 havingthe same fine pores of the first filter element 6. The filter element 8is provided with a hole and the outer diameter of one end of an overflowtube 9 is sealed in this hole so that the space between the two filterelements 6 and 8 is in communication with the portion 22 of thereplenishing chamber lying above the helium replenishing bath 5.Overflow tube 9 extends upwardly into portion 22.

The maximum level of the helium replenishing bath 5 is controlled by alevel sensor 10. The partition 4 is closed at its upper end by lid 11having a heat insulating layer 12. Bores 13, 14 and 15 pass through thelid 11 to provide entrance for the level sensor 10, a vacuumjacketsyphon 16 and its expansion valve 17, and a valve rod 18 for actuatingthe valve 7. As shown in detail in the drawings of U8. Pat. No.3,442,091, the vacuumjacket syphon 16 with its expansion valve 17 has aradiation protection jacket cooled by exhaust gas from the heliumreplenishing bath 5. That is, through this syphon not only liquid heliumis flowing into the replenishment chamber which is here schematicallyindicated by the arrow pointing downwards at the top of the figure butalso the cold gas evaporated from the helium replenishing bath 5 ispumped out through a tube connected to the radiation shield inside thejacket, which here is indicated by the direction of the two arrowspointing upwards at the top of the figure.

The level of the working bath 2 is controlled by schematicallyillustrated level sensor 19. The gas evaporating from the working bath 2is removed via exhaust gas line 20 and control valve 21.

During operation, the cryostat of the figure is first cooled with itsvalve 7 open, and filled with liquid helium. During this time, thecoupling of level sensor 10 to valve 17 is deactivated and liquid heliumoverflows from bath 5 into the overflow tube 9. Then the valve 7 isclosed and the pressure above the working bath 2 is brought down to thepressure corresponding to the desired working bath temperature below2.17 K ()\-point). In this way steady state operation is reached.

When the level of the working bath falls below the level sensor 19because of evaporation, the expansion valve 17 at the end of the syphon16 is opened with the help of a signal from sensor 19. Consequently,liquid helium begins to flow from a storage vessel downwards through theliquid carrying pipe in syphon 16. This flow is indicated by theuppermost arrow in the figure. This helium has an initial temperature of42 K. (boiling temperature of helium at one atmosphere pressure). Uponpassing through expansion valve 17, it expands due to the lower pressurepresent in portion 22 of the replenishing chamber and consequently itstemperature falls below 4.2 K. This helium collects above the upperfilter element 8 to form helium replenishing bath 5. When thereplenishing bath 5 reaches level sensor 10, expansion valve 17 closes.Upon the closing of expansion valve 17, the pressure in the portion 22of the replenishing chamber is reduced by means of a pump connected tothe exhaust gas line inside the vacuum jacket of syphon 16. Thispressure is reduced until the temperature of the replenishing bath 5reaches or falls below the preset temperature of the working bath 2.When this temperature is reached, the filter 8 becomes permeable to theliquid replenishing helium and the replenishing bath 5 begins to rundown into the space between the filters 6 and 8. As soon as a fewmillimeters of liquid helium have collected on top of the lower filter6, the filter 6 becomes permeable and the replenishing bath 5 runs thenthrough both filters 6 and 8 down into the working bath 2 and fills itup.

Regarding the flow of liquid helium from the replenishing bath 5 intothe working bath 2 at temperature or pressure equality on both sides ofthe filter 6 and at negative pressure differences (P is less than P thebehavior obtained is the same as that obtained using a single filter:the replenishment bath 5 flows downwards unhindered upon reaching theproper temperature. For positive pressure diflerences across filter 6 (Pis greater than P as indicated above, undesired liquid breakthrough canoccur when a single filter is used. However, as an advantage of thepresent invention, the double filter 6 and 8 remains completely shutuntil the proper temperature is reached in the replenishing bath 5,since no pressure difference exists across the filter element 8supporting the replenishing bath 5. Consequently, the danger ofdisturbance in the working bath below the filter by an undesired liquidbreakthrough because of a positive pressure diflerence is completelyeliminated. The operation of the apparatus of US. Pat. No. 3,442,091,consequently becomes more dependable.

It will be understood that the above description of the presentinvention is susceptible to various modifications, changes, andadaptations and that the same are intended to be comprehended within themeaning and range of equivalents of the appended claims.

We claim:

1. In apparatus for the continuous low-temperature cooling of objects ina helium II working bath;

which apparatus includes a working bath chamber for containing theworking bath, means for maintaining a reduced pressure in the workingbath chamber for regulating the temperature of the working bath, avacuum-jacketed syphon means for periodically replenishing the workingbath from a helium storage vessel, a replenishing chamber disposed atleast partially in said working chamber above the working bath andarranged for receiving the liquid helium delivered by the syphon means,said replenishing chamber including a first filter element forming atleast a portion of the lower extermity of said replenishing chamber andseparating the interior of said replenishing chamber from the workingbath, said first filter element being of a material which is at leastpartially impermeable to gaseous helium and helium I, as well as to anyhelium II having a temperature higher than that of the working bath, andmeans for maintaining a reduced pressure in said replenishing chamber;wherein the improvement comprises: a second filter element arranged insaid replenishing chamber above said first filter element and spacedtherefrom, said second filter element being of a material which is atleast partially impermeable to gaseous helium and helium I, as well asto any helium II having a temperature higher than that of the workingbath; and a communication means for placing the space between said firstand second filter elements in communication with the portion of saidreplenishing chamber above said second filter element.

2. An apparatus as claimed in claim 1, wherein said communication meansis an overflow tube sealed in a hole through said second filter elementand extending into the portion of said replenishing chamber above saidsecond filter element.

6 3. An apparatus as claimed in claim 2, further corn- References Citedprising a valve means in said first filter element for by- UNITED STATESPATENTS passmg said first filter element and a rod means extendingthrough said overflow tube for the selective opening and 3144210915/1969 KhPPmg 6t 62 55 closing of said valve means. 5

4. An apparatus as claimed in claim 2, further com- MEYER PERLIN PumaryExaminer prising means for sensing a maximum level of liquid R. C.CAPOSSELA, Assistant Examiner helium in the portion of said replenishingchamber above said second filter element, said overflow tube extendingUS. Cl. X.R. to a level above said maximum level. 10 174-15

